The present invention relates to a method of controlling an internal combustion engine connected to a torque converter having a lockup function. The invention also relates to a control unit for controlling an internal combustion engine. The invention furthermore relates to a vehicle drive train including such a control unit and a vehicle including such a drive train.
The invention is applicable on all vehicles having a torque converter with a lock up function. In particular the invention is applicable on working machines within the field of industrial construction machines, in particular wheel loaders and articulated haulers, where a transition between a torque converter mode and a lock up mode may be relatively frequent. Although the invention will be described with respect to a wheel loader, the invention is not restricted to this particular working machine, but may also be used in other working machines, such as dump trucks and trucks.
Vehicles provided with internal combustion engines connected to a torque converter are commonly known. Torque converters are used to transfer rotating power from the combustion engine to other drive train components of the vehicle.
A torque converter is able to multiply torque from the ingoing axle to the outgoing axle when there is a difference between the rotational speeds of the ingoing and outgoing axles. Hence torque converters are very useful especially for construction machines such as wheel loaders and articulated haulers for instance. However power is dissipated in a torque converter and the efficiency of torque converter is therefore limited. A typical efficiency curve defined in an efficiency/slip speed space displays a mid portion with an essentially flat curve characteristic with flanks with reduced efficiency. In the case the difference in rotational speed between ingoing and outgoing axles, which is commonly referred to as slip, is too small to allow operation of the torque converter at desired efficiency, it is common to allow engagement of a lock up coupling to mechanically connect the ingoing axle to the outgoing axle. It has shown to be suitable to operate the torque converter with a slip quota of between 0.5 and 0.8. This means that a quota of the rotational speed of the outgoing axle and the rotational speed of the ingoing axle is between 0.5 and 0.8.
The torque converter may thus include a lock up coupling for selective engagement or disengagement of a mechanical connection between said input and output shafts. The torque converter is a drive train component which transfers rotational power from a pump driven by an incoming axle to turbine connected to an outgoing axle via a stator which enables multiplication of the torque from the incoming axle to the outgoing axle while a slip is present. When the lock up coupling is engaged a mechanical connection between the incoming axle of the torque converter and the outgoing axle of the torque converter is present. The torque converter operates in lock up mode when the lock up coupling is engaged. When the lock up coupling is open no mechanical connection between the incoming axle of the torque converter and the outgoing axle of the torque converter is present.
Engagement or disengagement of the lock up function will have an impact on the load of the engine. When the lock up function is activated, i.e. the ingoing axle is mechanically connected to the outgoing axle of the torque converter, the transmission ratio over the torque converter is constant, preferably 1:1. In other words, in the lock up mode the torque converter function is deactivated.
When the lock up coupling is disengaged, variations in torque requirement for stable propulsion of the vehicle are met by variations on the slip in the torque converter. In this operation mode the primary object for the engine is to ensure that the engine operates at a stable engine speed requested by the driver. The combustion engine is therefore controlled to run at a desired engine speed in dependence of a position of an accelerator unit when a torque converter mode is present.
During the engagement process, the rotational speed of the engine will drop due to that a slip present before the engagement must be taken up until direct propulsion in engaged state will take place over the converter. If the engine speed is controlled in dependence of an operator input, the difference in rotational speed of the engine before the engagement process is initiated and after the engagement process is completed will be accounted for as a regulation error. Hence, a constant speed regulator will accelerate the engine at maximum torque.
When engagement of the lockup coupling take place engine load will increase simultaneously as the engine speed instantaneously drops due to that the engine is directly connected to the outgoing axle of the torque converter. The increased load of the engine together with a drop in engine speed will lead to an undesired behavior of the vehicle which will be subjected to a torque shock accelerating the vehicle.
Using state of the art engine speed regulation of the combustion engine will lead to acceleration of the vehicle at peak torque due to the lock up engagement, which leads to jerky behavior of the vehicle. The vehicle will therefore be difficult to control.
Attempts to reduce the torque shock resulting from engagement of a lock up clutch have been made in prior art. Many suggestions relate to adaptation of an EGR amount to allow reduction of the torque shock when engaging or disengaging the lock up clutch.
In U.S. Pat. No. 4,716,999 it is suggested to operate an air intake control valve in order to reduce the torque shock. It is suggested to close the control valve to reduce the amount of air to reduce the engine speed without decreasing the engine torque when engagement of the lock up clutch occurs.
In a corresponding manner disengagement of the lock up clutch leads to a sharp drop in load of the engine. The engine will simultaneously increase its speed due to the lowered load.
Even though various attempts to mitigate the occurrence of torque shock when engaging a lock up clutch have been made further improvements are necessary to provide method for controlling a combustion engine enabling good drive comfort during engagement and disengagement of the lock up function.
It is desirable to provide method for controlling a combustion engine enabling improved drive comfort during activation of a torque converter mode or activation of a lock up mode by engagement and disengagement of a lock up function.
The invention relates, according to an aspect thereof, to a method of controlling an internal combustion engine connected to a torque converter being arranged to operate in a torque converter mode or a lock up mode. According to an aspect of the inventive method it is first determined whether the torque converter mode or the lock up mode is activated. Depending on which mode the torque converter is operated in, a selection of how to control the combustion engine is made. When the lock up mode is present a control unit is set to control an engine output power and/or engine output torque in dependence of an operator input. When the torque converter mode is present, the control unit is set to select to control an engine speed in dependence of the operator input.
By controlling the output power and/or output torque of the combustion engine in dependence of an operator input, such as the position of an accelerator unit, when the lock up mode is present, the difference in engine speed is not dealt with as a regulation error which must be compensated by regulating the torque of the engine in order to reach the desired engine speed. Instead, the engine is controlled to deliver a requested power and/or torque. An undesired acceleration of the engine may therefore be avoided.
The operator input may be generated by an operator input to an accelerator unit such as an accelerator pedal, a manual lever or any other control instrument where an operator may control the operation of the engine.
In the lock up mode an amount of injected fuel may be regulated to provide a desired engine output power and/or torque as indicated by said operator input The operator input may be a position of an accelerator pedal or manual lever or similar. Once the operator input is determined, for instance via a sensor determining the position of the accelerator unit, a desired engine output power and/or torque may be determined by use of a map, a look up table or a function. Hence, the desired engine output power and/or torque may be determined from a present position of the accelerator unit.
In the torque converter mode an amount of injected fuel may be regulated to provide a desired engine speed indicated by said operator input to the accelerator unit. The operator input to the accelerator unit may be a position of an accelerator pedal or manual lever. Once the operator input is determined, for instance via a sensor determining the position of the accelerator unit, a desired engine speed may be determined by use of a map, a look up table or a function. Hence, the desired engine speed may be determined from a present position of the accelerator unit.
The invention furthermore relates to a control unit for controlling an internal combustion engine connected to a torque converter being arranged to operate in a torque converter mode or a lock up mode. The control unit includes a torque converter operation control block arranged to determine whether the torque converter mode or the lock up mode is activated. The control unit is arranged to control an engine output power and/or engine output torque in dependence of an operator input when said lock up mode is present. In the event the torque converter mode is present, the control unit is arranged to select to control an engine speed in dependence of operator input.
The merits of the control unit, the embodiments thereof, the vehicle drive train and the vehicle according to the invention are apparent from the description of the method above and from the description of the embodiments of the invention below.